Reading your texts for fun and profit – how criminals subvert SMS-based MFA

3 August 2017

Why multi factor?

Read almost any cyber security related news and you will start to see why using a password alone isn’t the most secure way of preventing unauthorized access to your account. Multi-factor authentication (MFA) is invaluable because it adds extra obstacles for attackers attempting to access your account, hence why it has become such a popular account security control. There are different flavors of MFA ranging from codes sent via text (SMS), authentication applications, or physical devices. Naturally attackers are going to try and circumvent MFA, so we conducted some research into the ways SMS-based MFA could be subverted, which are outlined below.

Threats to SMS based solutions

Recently we came across a service that claimed to provide customers with the ability to redirect phone calls and text messages, advertised on at least one hacking forum for over a year and hosted on the Tor network (see Figure 1). Named “Interconnector” and offering “SS7 Services”, this was probably in reference to what is known as Signaling System No. 7 (SS7), a signaling language used to ensure that the networks of telecommunication companies can interoperate. For example, SS7 allows someone in one country to send messages to someone in another country. If this Interconnector service was genuine (although many forum users claimed it was a scam), it would almost certainly be deemed as valuable for threat actors. Why? Because the ability to intercept SMS messages would circumvent MFA protection which relies on tokens sent via this channel. This might include your social media accounts and your online bank accounts. It might even be used to authenticate online transactions.

 

interconnector service

Figure 1 - the "Interconnector" SS7 dark web service

The abuse of SS7 for this purpose isn’t a pipe dream, there are at least a few examples of it being used maliciously in the past. In May 2017, it was reported that criminals had been able to access and steal funds from compromised bank accounts by redirecting SMS messages containing one time tokens and mobile transaction authorization numbers. There was also reporting in 2014 that a number of Ukrainian mobile subscribers’ phone calls had been redirected as a result of custom SS7 packets. The goal of the redirection is unknown at the time of writing.

Although SS7 abuse is certainly interesting, MFA tokens can also be obtained via other means. For example, the Retefe banking trojan was used alongside mobile malware to harvest SMS codes, while the Dridex banking trojan harvested these codes and its operators used them in real time. Threat actors can also redirect messages and calls to different SIM cards; Wired published a report in June in which it claimed attackers were able to socially engineer employees at a telecommunications company in order to have a target’s calls and text messages redirected – otherwise known as “SIM Swapping”. Furthermore, so called “fake” mobile towers, or International Mobile Subscriber (IMSI) catchers, could also be used to intercept mobile traffic.

All of these methods have their own limitations or requirements to be successful. All of them, for example, require an attacker to first obtain the relevant account credentials before they can consider intercepting MFA tokens. Furthermore, many of the examples we’ve highlighted in this blog require a relatively large amount of effort for the threat actors involved. Of these then, the abuse of SS7 would be most likely to be viable at scale, but the exact level of access and capability required to achieve such an attack isn’t entirely clear.

Considering the alternatives

All of these methods and their use in the wild show that SMS-based two factor authentication (2FA) is not infallible and that criminals have an interest in circumventing it. In this context, it is unsurprising that NIST recommended in 2016 that “M” in MFA should not be SMS. 

Each method has its own advantages and disadvantages as well as capability requirements. However, someone with the appropriate capability and intent could successfully capture SMS-based MFA codes in order to access accounts, or conduct fraudulent transfers. There are some basic mitigation steps individuals could follow, including:

  1. Not clicking on links in suspicious or unsolicited emails or text messages
  2. Avoiding the download of mobile applications or games from unofficial stores
  3. Operate anti-virus solutions and keep them up to date
  4. Considering the use of alternative MFA such as authenticators such as hard tokens